diff --git a/README.md b/README.md
index 6604a79b4..808593fb3 100644
--- a/README.md
+++ b/README.md
@@ -1986,6 +1986,33 @@ Since Puppet 4.0.0, you can use the + operator to achieve the same merge.
 
     $merged_hash = $hash1 + $hash2
 
+If merge is given a single `Iterable` (`Array`, `Hash`, etc.) it will call a given block with
+up to three parameters, and merge each resulting Hash into the accumulated result. All other types
+of values returned from the block (typically `undef`) are skipped (not merged).
+
+The codeblock can take 2 or three parameters:
+* with two, it gets the current hash (as built to this point), and each value (for hash the value is a [key, value] tuple)
+* with three, it gets the current hash (as built to this point), the key/index of each value, and then the value
+
+If the iterable is empty, or no hash was returned from the given block, an empty hash is returned. In the given block, a call to `next()`
+will skip that entry, and a call to `break()` will end the iteration.
+
+*Example: counting occurrences of strings in an array*
+```puppet
+['a', 'b', 'c', 'c', 'd', 'b'].merge | $hsh, $v | { { $v => $hsh[$v].lest || { 0 } + 1 } }
+# would result in { a => 1, b => 2, c => 2, d => 1 }
+```
+
+*Example: skipping values for entries that are longer than 1 char*
+
+```puppet
+['a', 'b', 'c', 'c', 'd', 'b', 'blah', 'blah'].merge | $hsh, $v | { if $v =~ String[1,1] { { $v => $hsh[$v].lest || { 0 } + 1 } } }
+# would result in { a => 1, b => 2, c => 2, d => 1 } since 'blah' is longer than 2 chars
+```
+
+The iterative `merge()` has an advantage over doing the same with a general `reduce()` in that the constructed hash
+does not have to be copied in each iteration and thus will perform much better with large inputs.
+
 *Type*: rvalue.
 
 #### `min`
diff --git a/lib/puppet/functions/merge.rb b/lib/puppet/functions/merge.rb
new file mode 100644
index 000000000..62ddf99d0
--- /dev/null
+++ b/lib/puppet/functions/merge.rb
@@ -0,0 +1,94 @@
+# Merges two or more hashes together or hashes resulting from iteration, and returns the resulting hash.
+#
+# @example Using merge()
+#
+#   $hash1 = {'one' => 1, 'two', => 2}
+#   $hash2 = {'two' => 'dos', 'three', => 'tres'}
+#   $merged_hash = merge($hash1, $hash2)
+#   # The resulting hash is equivalent to:
+#   # $merged_hash =  {'one' => 1, 'two' => 'dos', 'three' => 'tres'}
+#
+# When there is a duplicate key, the key in the rightmost hash will "win."
+#
+# Note that since Puppet 4.0.0 the same merge can be achieved with the + operator.
+#
+#    $merged_hash = $hash1 + $hash2
+#
+# If merge is given a single Iterable (Array, Hash, etc.) it will call a given block with
+# up to three parameters, and merge each resulting Hash into the accumulated result. All other types
+# of values returned from the block (typically undef) are skipped (not merged).
+#
+# The codeblock can take 2 or three parameters:
+# * with two, it gets the current hash (as built to this point), and each value (for hash the value is a [key, value] tuple)
+# * with three, it gets the current hash (as built to this point), the key/index of each value, and then the value
+#
+# If the iterable is empty, or no hash was returned from the given block, an empty hash is returned. In the given block, a call to `next()`
+# will skip that entry, and a call to `break()` will end the iteration.
+#
+# @example counting occurrences of strings in an array
+#   ['a', 'b', 'c', 'c', 'd', 'b'].merge | $hsh, $v | { { $v => $hsh[$v].lest || { 0 } + 1 } }
+#   # would result in { a => 1, b => 2, c => 2, d => 1 }
+#
+# @example skipping values for entries that are longer than 1 char
+#   ['a', 'b', 'c', 'c', 'd', 'b', 'blah', 'blah'].merge | $hsh, $v | { if $v =~ String[1,1] { { $v => $hsh[$v].lest || { 0 } + 1 } } }
+#   # would result in { a => 1, b => 2, c => 2, d => 1 } since 'blah' is longer than 2 chars
+#
+# The iterative `merge()` has an advantage over doing the same with a general `reduce()` in that the constructed hash
+# does not have to be copied in each iteration and thus will perform much better with large inputs.
+#
+Puppet::Functions.create_function(:merge) do
+  dispatch :merge2hashes do
+    repeated_param 'Variant[Hash, Undef, String[0,0]]', :args # this strange type is backwards compatible
+    return_type 'Hash'
+  end
+
+  dispatch :merge_iterable3 do
+    repeated_param 'Iterable', :args
+    block_param 'Callable[3,3]', :block
+    return_type 'Hash'
+  end
+
+  dispatch :merge_iterable2 do
+    repeated_param 'Iterable', :args
+    block_param 'Callable[2,2]', :block
+    return_type 'Hash'
+  end
+
+  def merge2hashes(*hashes)
+    accumulator = {}
+    hashes.each { |h| accumulator.merge!(h) if h.is_a?(Hash) }
+    accumulator
+  end
+
+  def merge_iterable2(iterable)
+    accumulator = {}
+    enum = Puppet::Pops::Types::Iterable.asserted_iterable(self, iterable)
+    enum.each do |v|
+      r = yield(accumulator, v)
+      accumulator.merge!(r) if r.is_a?(Hash)
+    end
+    accumulator
+  end
+
+  def merge_iterable3(iterable)
+    accumulator = {}
+    enum = Puppet::Pops::Types::Iterable.asserted_iterable(self, iterable)
+    if enum.hash_style?
+      enum.each do |entry|
+        r = yield(accumulator, *entry)
+        accumulator.merge!(r) if r.is_a?(Hash)
+      end
+    else
+      begin
+        index = 0
+        loop do
+          r = yield(accumulator, index, enum.next)
+          accumulator.merge!(r) if r.is_a?(Hash)
+          index += 1
+        end
+      rescue StopIteration # rubocop:disable Lint/HandleExceptions
+      end
+    end
+    accumulator
+  end
+end
diff --git a/spec/functions/merge_spec.rb b/spec/functions/merge_spec.rb
index 5f9747282..7bffaab85 100644
--- a/spec/functions/merge_spec.rb
+++ b/spec/functions/merge_spec.rb
@@ -2,20 +2,34 @@
 
 describe 'merge' do
   it { is_expected.not_to eq(nil) }
-  it { is_expected.to run.with_params.and_raise_error(Puppet::ParseError, %r{wrong number of arguments}i) }
-  it { is_expected.to run.with_params({}, 'two').and_raise_error(Puppet::ParseError, %r{unexpected argument type String}) }
-  it { is_expected.to run.with_params({}, 1).and_raise_error(Puppet::ParseError, %r{unexpected argument type (Fixnum|Integer)}) }
-  it { is_expected.to run.with_params({ 'one' => 1, 'three' => { 'four' => 4 } }, 'two' => 'dos', 'three' => { 'five' => 5 }).and_return('one' => 1, 'three' => { 'five' => 5 }, 'two' => 'dos') }
-
   it {
-    pending 'should not special case this'
-    is_expected.to run.with_params({}).and_return({})
+    is_expected.to run \
+      .with_params({}, 'two') \
+      .and_raise_error(
+        ArgumentError, \
+        Regexp.new(Regexp.escape("rejected: parameter 'args' expects a value of type Undef, Hash, or String[0, 0], got String")),
+      )
+  }
+  it {
+    is_expected.to run \
+      .with_params({}, 1) \
+      .and_raise_error(ArgumentError, %r{parameter 'args' expects a value of type Undef, Hash, or String, got Integer})
+  }
+  it {
+    is_expected.to run \
+      .with_params({ 'one' => 1, 'three' => { 'four' => 4 } }, 'two' => 'dos', 'three' => { 'five' => 5 }) \
+      .and_return('one' => 1, 'three' => { 'five' => 5 }, 'two' => 'dos')
   }
+
+  it { is_expected.to run.with_params.and_return({}) }
+  it { is_expected.to run.with_params({}).and_return({}) }
   it { is_expected.to run.with_params({}, {}).and_return({}) }
   it { is_expected.to run.with_params({}, {}, {}).and_return({}) }
+
   describe 'should accept empty strings as puppet undef' do
     it { is_expected.to run.with_params({}, '').and_return({}) }
   end
+
   it { is_expected.to run.with_params({ 'key' => 'value' }, {}).and_return('key' => 'value') }
   it { is_expected.to run.with_params({}, 'key' => 'value').and_return('key' => 'value') }
   it { is_expected.to run.with_params({ 'key' => 'value1' }, 'key' => 'value2').and_return('key' => 'value2') }
@@ -24,4 +38,43 @@
       .with_params({ 'key1' => 'value1' }, { 'key2' => 'value2' }, 'key3' => 'value3') \
       .and_return('key1' => 'value1', 'key2' => 'value2', 'key3' => 'value3')
   }
+  describe 'should accept iterable and merge produced hashes' do
+    it {
+      is_expected.to run \
+        .with_params([1, 2, 3]) \
+        .with_lambda { |_hsh, val| { val => val } } \
+        .and_return(1 => 1, 2 => 2, 3 => 3)
+    }
+    it {
+      is_expected.to run \
+        .with_params([1, 2, 3]) \
+        .with_lambda { |_hsh, val| { val => val } unless val == 2 } \
+        .and_return(1 => 1, 3 => 3)
+    }
+    it {
+      is_expected.to run \
+        .with_params([1, 2, 3]) \
+        # rubocop:disable Style/Semicolon
+        .with_lambda { |_hsh, val| raise StopIteration if val == 3; { val => val } } \
+        .and_return(1 => 1, 2 => 2)
+    }
+    it {
+      is_expected.to run \
+        .with_params(['a', 'b', 'b', 'c', 'b']) \
+        .with_lambda { |hsh, val| { val => (hsh[val] || 0) + 1 } } \
+        .and_return('a' => 1, 'b' => 3, 'c' => 1)
+    }
+    it {
+      is_expected.to run \
+        .with_params(['a', 'b', 'c']) \
+        .with_lambda { |_hsh, idx, val| { idx => val } } \
+        .and_return(0 => 'a', 1 => 'b', 2 => 'c')
+    }
+    it {
+      is_expected.to run \
+        .with_params('a' => 'A', 'b' => 'B', 'c' => 'C') \
+        .with_lambda { |_hsh, key, val| { key => "#{key}#{val}" } } \
+        .and_return('a' => 'aA', 'b' => 'bB', 'c' => 'cC')
+    }
+  end
 end